PROJECT SUMMARY/ABSTRACT Glucocorticoid (GC) steroids are commonly prescribed for a number of inflammatory conditions like arthritis, asthma, and muscle disease, but chronic use can lead to side effects including muscle wasting and weakness. GC steroid-mediated muscle atrophy is driven by upregulation of atrophy factors like atrogin-1 and MuRF-1. Acute GC steroid treatment such as once-weekly dosing can improve muscle performance while avoiding stimulation of these atrophy factors. GC steroids function through the glucocorticoid receptor (GR), which binds to the steroid ligand and then translocates into the nucleus to activate or repress its target genes. The primary drivers of GR targeting are co-factors that facilitate DNA binding, but the precise co-factors in skeletal muscle responsible for the beneficial impact of weekly dosing have not been determined. The identification of specific co-factors would allow for a focused treatment strategy that would enhance efficacy of GC steroids while reducing the detrimental side effects of muscle wasting. Preliminary data in this proposal show that, in wildtype muscle, chronic daily GC steroid exposure decreased muscle function while once-weekly GC dosing improved muscle function and upregulated pro-ergogenic genes. Intriguingly, we found separate cohorts of pro-ergogenic genes upregulated in males versus females, suggesting that different mechanisms are responsible for the benefits of weekly GC dosing in each sex. This work will test the hypothesis that the skeletal muscle responsiveness to weekly GC steroid treatment is modulated by sex. This project will utilize both physiological assays and molecular studies to determine how different pathways in males and females impact muscle performance in response to GC stimulus. RNAseq and ChIPseq will be used to identify loci that are divergently regulated by GR in skeletal muscle of male and female mice. Co-immunoprecipitation and ChIP-qPCR will be used to identify the co-factors responsible for the distinct regulation of these loci in each sex. Muscle function and endurance assays will be used to investigate how muscle performance is divergently enhanced by these transcriptional differences. Together, the aims of this proposal will interrogate the mechanism of GR targeting in a sexually dimorphic system in order to understand the role that GR co-factors play as promotors of beneficial muscle remodeling in males and females. The results of this study are expected to provide key insight into the mechanisms of GC steroid action in muscle yielding a valuable genomic signature to help translate these findings to help the nearly one million Americans currently on chronic GC steroid treatment and subject to the side effects of this therapy.